While battery technology has come a long way in recent years, it still places constraints on the size and shape of cellphones and other personal electronic devices. Researchers at Rice University are looking to solve this problem by developing a rechargeable lithium-ion battery that can be painted on virtually any surface ... and early results are promising.

Lithium-ion batteries are, in essence, a collection of five different layers, each with a specific function, that need to be in contact with each other to work properly. To maximize the area of contact the layers are usually tightly rolled up in a spiral, meaning the battery nearly always ends up shaped either as a cylinder or a rectangle, which limits the form factor in many electronic devices.

A team led by Rice materials scientist Pulickel Ajayan has managed to reduce the five layers that characterize a lithium-ion battery (two collectors, two electrodes, and a separator) to a spray paint made from different materials that has the same functionality.

The researchers have shown this "paint" to be effective on a variety of surfaces - glass, stainless steel, glazed ceramic tiles and flexible polymer sheets.

In one demonstration, nine bathroom tile-based batteries were connected in parallel and the middle tile topped by a solar cell. Once fully charged, the tiles powered a LED array for six hours with a steady 2.4 volts. The batteries were also put through 60 charge-discharge cycles and exhibited only a very small drop in capacity.

The researchers have filed for a patent and are working on scaling up the spray-on batteries to improve performance. The possibility of integrating the device with paintable solar cells to create a very unique energy-harvesting solution is also being investigated. To achieve this, the team is now looking for electrolytes that would make the system work in the open air.